This invention relates to a fuel system for delivering fuel to gas turbine engines.
Prior art fuel systems include a large positive displacement pump driven by a turbine engine through a gearbox. To achieve the desired fuel volumes and pressures for the demands of the turbine engine, the positive displacement pump is specifically oversized for peak demand. As a result, the fuel system utilizes a bypass valve to return the unneeded fuel back to tank, which is greatly inefficient. Moreover, the returned fuel is hot, which undesirably raises the temperature of the fuel within the fuel tank. Excess pressure that is generated must be relieved using a pressure regulator to mitigate any potential burst or over pressure conditions.
Prior art fuel systems employ precision metering valves to regulate the flow rate through the fuel system to meet the turbine engine fuel demand. The metering valves have tight tolerances and any contamination present within the fuel system comprises the operation of the valves.
Utilizing an electric motor instead of the turbine engine to drive the positive displacement pump to meet a turbine engine fuel demand is not practical. In one example, an 80 horsepower motor weighing over 100 pounds would be necessary to supply the needed fuel to the turbine engine, which is not acceptable within the industry.
Another problem with prior art fuel systems is a phenomena known as “coking.” A fuel manifold distributes fuel to multiple nozzles associated with the turbine engine. During shutdown, residual fuel in the fuel manifold can drain into the lower nozzles and burner system and form carbon build up or coke. Coking results in reduced burner efficiency, nozzle flow irregularities, combustion instability and excessive emissions as un-combusted fuel is vaporized. To address this problem, fuel storage canisters have been used that rely on complicated timing valves and turbine burner pressure, which is considered unreliable.
What is needed is a fuel system that reduces coking while eliminating the need for expensive, high accuracy fuel metering valves and/or reduces excess pressure and flow within the fuel system.